1. Field of the Invention
The present invention relates to a braking force controller for an electric vehicle, and particularly, to a braking force controller that uses the regenerative braking force of a motor of the vehicle.
2. Description of the Related Art
Some electric vehicles employ a hydraulic brake, such as the ones used in ordinary gasoline-powered vehicles, as an anti-lock brake, and stops a regenerative braking action of a motor of the vehicle during an anti-lock braking operation. This is disadvantageous because the best features of an electric vehicle, i.e., using braking forces as energy to charge a battery, is wasted.
Various anti-lock braking techniques using the regenerative braking action of a vehicle motor have been proposed. For example, Japanese Unexamined Patent Publication (Kokai) No. 48-2515 (a first prior art) detects the speed of a driving wheel, estimates a vehicle speed and a slip ratio according to the detected speed, and controls regenerative braking force according to the vehicle speed and slip ratio. Japanese Unexamined Utility Model Publication (Kokai) No. 5-2501 (a second prior art) detects a difference between the speeds of driving and driven wheels, calculates a slip ratio according to the difference, and controls regenerative braking force according to the slip ratio.
The first prior art is inaccurate in estimating a vehicle speed according to a driving wheel speed, so that it is incapable of regeneratively braking a vehicle at an optimum slip ratio of about 0.2 at which wheel-to-road friction becomes maximum. The second prior art is also inaccurate in detecting a vehicle speed because a wheel speed does not correctly correspond to a vehicle speed when mechanical braking force on driven wheels is large, so that the second prior art is also incapable of regeneratively braking a vehicle at an optimum slip ratio.